Large-amplitude transfer motion of hydrated excess protons mapped by ultrafast 2D IR spectroscopy

In recent years, vibrational spectroscopy has been homing in on how water accommodates dissolved protons in acidic solution. Most such studies have examined adjacent stretching or bending modes. Dahms et al. pinpoint the vibrational dynamics of the acidic proton itself, which is sandwiched between two water molecules in a so-called Zundel motif. Comparing spectra in bulk water and in acetonitrile (a known Zundel host) revealed the persistence of this motif in aqueous acid on a subpicosecond time scale. Persistence is sustained by the hydrogen-bonding network among the surrounding water molecules.